Petroleum is produced from subterranean formations through wells penetrating the formation. Initially, natural reservoir pressure may be sufficient to cause the oil to flow to the surface. As natural reservoir pressure declines due to production from the reservoir, various forms of artificial lift may be employed to bring to the surface the oil which flows into the well bores. This phase of production is usually termed "primary".
Typically, during the primary production period of a reservoir, only 10-30% of the oil initially in place may be economically recovered. As the production rate from a reservoir falls near the level at which it is no longer economic to operate the field, active steps, usually termed "secondary", are taken to increase recovery. In some instances, secondary recovery techniques may be employed from the time of initial production from a reservoir to optimize recovery.
The most common form of secondary recovery is the process known as waterflooding. In a waterflood project, water is injected into the oil-producing formation through injection wells, repressurizing the formation and sweeping oil which would not have otherwise been produced into production wells. Such a procedure will usually allow the economic production of an additional 10-30% of the oil originally in place.
As is readily seen, a reservoir may have as much as 80% by volume of the original oil in place still unproduced after primary and secondary operations have terminated. The increasing scarcity of crude oil has led to the development of various techniques intended to allow further production from reservoirs which have been depleted by primary and secondary operations. These operations aimed at enhanced oil recovery (EOR) from conventionally depleted reservoirs are generally termed "tertiary".
Among the most promising of tertiary recovery methods are processes wherein a solution of surfactants is injected into the reservoir. The surfactant mobilizes the oil remaining in place after conventional production and allows it to be swept into production wells.
Many types of surfactants have been investigated and the choice of which surfactants to employ in a chemical enhanced oil recovery operation is dependent upon reservoir characteristics as well as the cost and availability of the surfactants. One problem with many surfactants is their high cost of manufacture. Accordingly, surfactants which are relatively cheap have an inherent advantage in the market place.
Lignin is a waste by-product that the pulping industry produces in prodigious amounts. As a result, a large research effort has been undertaken over the last forty (40) years in attempts to find uses for the large volume of lignin by-product.
Lignin itself is a major noncarbohydrate constituent of wood and woody plants. It functions as a natural plastic binder for the cellulose fibers and permeates the membranes of the cells and the spaces between cells of the wood, thereby strengthening them. Although many investigators have attempted to define lignin in terms of its precise chemical structure, there has been no success in proposing an adequate definition. It should be noted that lignin is not a single definable compound, but rather is a group or system of high molecular weight, amorphous, materials that are chemically closely related. Furthermore, it should be noted that lignin, in its natural form in wood, is primarily of academic interest only. Pure and natural lignin has resisted all efforts at isolation in its completely and unchanged state. That is, no method has yet been devised for isolation of lignin of wood in a form identical with that in which it exists in its natural state. One description of lignin is that it is comprised of polymeric chains having molecular weights over 10,000 with multiple different units. One substantial monomeric unit contained in the lignin polymeric chains is propyl phenol.
According to U.S. Pat. No. 4,548,721, lignins are all characterized by having little or no oil solubility. This reference goes on to state that test results have shown these lignins to be ineffective when used as surfactants in enhanced oil recovery processes. Accordingly, much research has been expended at the modification of such lignins in order to produce oil-soluble derivatives thereof which, when used in conjunction with water-soluble surfactants produce comparable results in efficiency of enhanced oil recovery to that achievable using a conventional surfactant solution of one (1) or more costly petroleum sulfonates.
U.S. Pat. Nos. 4,739,041 and 4,790,382 describe a method of producing surfactants from lignin which comprises subjecting lignin to two reactions, alkylation and oxidation. The lignin surfactants are used in surfactant floods for enhanced oil recovery. However, when preparing surfactant systems with divalent ion brines, these lignin derivatives must be dissolved in fresh water followed by the addition of the other compounds and brine. They will not dissolve in divalent brines alone. They must first be solubilized in fresh water before adding other components. In many cases, this is a draw back since fresh water is not readily available in large quantities in the field.
U.S. Pat. Nos. 4,739,040 and 4,787,454 disclose a method of surfactant flooding with lignin surfactants produced by reducing lignin in the presence of a carbon monoxide or hydrogen reducing agent at high temperature and pressure to produce low molecular weight lignin phenols, and subjecting the lignin phenols to one or a combination of several reactions such as alkoxylation, alkylation, sulfonation, sulfation, alkoxysulfation and sulfomethylation.
U.S. Pat. No. 5,094,295 discloses a method of producing water-soluble surfactants from lignin, which comprises alkylating lignin at phenolic oxygen sites with an alkyl chain having about 3 to about 24 carbon atoms, sulfonating the alkylated lignin, and oxidizing the alkylated, sulfonated lignin sufficiently to break the lignin into smaller polymeric and monomeric compounds having water-soluble surfactant properties. These modified lignin surfactants are used in surfactant floods for enhanced oil recovery.
U.S. Pat. Nos. 4,756,370 and 4,821,803 involve substituting lignosulfonate and amine into a blended surfactant system for a high equivalent weight, oil-soluble petroleum sulfonate. U.S. Pat. No. 4,548,721 involves using the reaction product of a lignin and amine at elevated temperatures as an enhanced oil recovery surfactant. U.S. Pat. Nos. 4,822,501 and 5,114,599 involve substituting amine and a Kraft lignin chemically modified by ozonolysis or sulfonation into a blended surfactant system for a high equivalent weight, oil-soluble sulfonate.
Others have shied away from the use of unmodified Kraft lignin for use in surfactant systems for enhanced oil recovery purposes. Only lignosulfonates or chemically modified Kraft lignins have been considered suitable for use in this type of application. Most pulping mills world-wide use the Kraft pulping process. The ability to use unmodified Kraft lignin in surfactant systems would eliminate the need to construct special facilities to modify Kraft lignins of the world where lignosulfonates are unavailable. Currently, there is a need to eliminate this costly step of modification and improve the economics of surfactant flooding.